well, i'm nigh on 20 now (couple months to go) living at home with my father, and have a younger brother (12) who wants to play.
the best advice i can give to parents, get involved with the scene or someone at least somewhat familiar with it before going out with your child.
going to a field on 'new players night' (most fields will have one at least once a month) will help. be sure to learn all you can.
other good advice, as stated above, avoid those who have all the latest gear, they are not as liable to be helpful as the guy in his late 20's who is sporting the beat up but well cared for gear. every time i go to a field, as an experienced player, i try to align myself with other players who share a similar mentality.
my parents never supported me taking part in any 'violent' sports, i wasn;t able to shoot archery until i bought my own bow, was never allowed to do martial arts unless i was the one paying for classes and when i wanted to play paintball, i was told flat out "NO".
i cannot stress how important it is to support your kid's interests in this case, they have to show a willingness to help out and be responsable, but they really do need to be supported in some fashion (taking one for the team)